which led in the arduino uno microcontroller blinks when it transmits data?

 On the Arduino Uno, there isn't a dedicated LED that blinks specifically for data transmission (like serial communication). However, there is one that blinks for data reception, and the confusion is very common.

Let me clarify which LEDs do what:

The LEDs on Arduino Uno

1. TX LED (Transmit) - This is the main answer

• Location: Labeled "TX" near digital pin 1 (D1)

• What it does: This LED blinks when the Arduino Uno transmits data to the computer via USB serial.

• When you'll see it blink: When your code uses Serial.print() or Serial.println() to send data to the Serial Monitor.

2. RX LED (Receive)

• Location: Labeled "RX" near digital pin 0 (D0)

• What it does: This LED blinks when the Arduino Uno receives data from the computer.

• When you'll see it blink: When you send commands from the Serial Monitor to your Arduino.

3. Built-in LED (Pin 13)

• Location: Labeled "L" on the board

• What it does: Connected to digital pin 13. It's a general-purpose LED for debugging.

• Famous behavior: This is the LED that blinks in the classic "Blink" example sketch.

Practical Example

Upload this code to see all three LEDs in action:

cpp

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);  // Pin 13 LED
  Serial.begin(9600);            // Start serial communication
  Serial.println("Arduino started!"); // This will make TX LED blink
}

void loop() {
  // Blink the built-in LED (pin 13)
  digitalWrite(LED_BUILTIN, HIGH);
  delay(500);
  digitalWrite(LED_BUILTIN, LOW);
  delay(500);
  
  // Send data to Serial Monitor - TX LED will blink
  Serial.println("Hello from Arduino!");
  delay(1000);
  
  // Check if data is received - if you type in Serial Monitor, RX LED will blink
  if (Serial.available()) {
    String received = Serial.readString();
    Serial.print("Received: ");
    Serial.println(received);  // TX LED will blink again
  }
}

What You'll Observe:

1. The "L" LED (pin 13) will blink steadily every second

2. The TX LED will blink briefly each time Serial.println("Hello from Arduino!") executes

3. The RX LED will blink if you type something in the Serial Monitor and press Send

Quick Summary Table:

LED	Label	Purpose	Blinks When...
TX	TX	Data Transmission	Arduino sends data to computer
RX	RX	Data Reception	Arduino receives data from computer
Built-in	L	General purpose	You control digital pin 13

So to directly answer your question: The TX LED blinks when the Arduino Uno transmits data.